Do you Twitter?
Twitter is a new free service that is catching up really well, it took me a while to get the concept behind it.
Simply it is a place where you create an account (you can add your cell number too), invite friends to join.
No once everything is set you can send messages to Twitter either online, from a desktop client (Twitterrific), or SMS, the message will be forwarded to everyone on the list, if they activate their phone with the service, an SMS will reach them and they can reply back.
Now that I got the concept, I am starting to like it, I set it up to send all my friends messages about when we will gather, saves money instead of sending multiple SMSs. 
now the whole thing seems like SMS walki talki. everyone is getting the same message.
Update: It is a bad idea, it can drive you mad, don’t use Twitter. 
I won’t be held responsible if you do.